This is a Brake Booster & Master Cylinder out of a E39 530. This master cylinder had been leaking brake fluid for a while. The brake fluid leaked into the brake booster, causing the booster to start to rust on the inside. Symptoms of a leaking master cylinder are a very soft brake pedal, that travels to the floor. A brake booster going bad can cause a very stiff brake pedal. This vehicle had a strange combination of both.